The present invention relates generally to robotic painting systems and in particular to a robotic apparatus for applying electrically conductive paint to the external surfaces of automotive vehicle bodies.
Prior art paint booths are well known. A typical prior art paint booth, used to paint the exterior surfaces of vehicle bodies in both continuous conveyance and stop station systems, includes an enclosure housing a plurality of paint applicators. In one configuration, the applicators are mounted on an inverted U-shaped support structure that includes two vertical supports, one on either side of the path of travel of the bodies, connected at their tops by a horizontal support. This support structure is used to paint the top surfaces of the body and the horizontal beam can be fixed or can have an additional degree of freedom to move along the top of the vehicle body being painted. Another painting device is used in the same painting zone to paint the sides of the body and generally does not have the capability to move laterally along the length of the body. Disadvantages of this type of painting apparatus include lack of flexibility to provide optimized standoff distance between the body surface and the applicator along with inefficient use of the allotted painting cycle time. In the case of the top surface painting machine, the paint applicators are mounted on a common beam: therefore, the distance between each paint applicator and the surface to be painted varies with the contours of the vehicle body. In the case of the side painting machine, the paint applicators do not move transverse to the path of the vehicle body. They can only paint the portion of the body that is in front of the applicator leaving a good portion of the available cycle time unused.
A more recent alternative to the support structure has been floor-mounted robots disposed along the sides of the painting booth. The robots mount either spray guns or rotary applicators (bell machines) for directing atomized paint toward the vehicle body.
While rotary applicators have advantages over spray guns, there are some associated disadvantages. The prior art floor mounted robots, especially bell machines, are inherently very costly and limit visual access to the spray booth. The bell machines require more bells for the same throughput due to limited orientation capability. The additional bells use more paint per vehicle due to per bell paint waste during color changing. Prior art floor mounted robots also require significant booth modification when installed in existing paint booths, increasing installation time and cost, and require more booth length and width. The rail axis of floor mounted robots requires doors at both ends of the booth. The waist axis of the floor mounted robot requires an additional safety zone at the ends of the spray booth and the rail cabinets of the floor mounted robots encroach into the aisle space. Floor mounted robots also require frequent cleaning due to the down draft of paint overspray causing paint accumulation on the robot arm and base, which results in higher maintenance and cleaning costs.
The prior art bell zone machines also lack flexibility. Additional and more flexible robot zones are required because the prior art machines unable to reach substantially all paintable surfaces on one side of the body and, therefore, have limited backup capability for an inoperative painting machine. Additional robot zones are also used to provide backup capability for the less flexible prior art painting machine.
It is desirable, therefore, to provide a painting apparatus and a painting system that utilizes robots in an efficient and cost-effective manner that minimizes paint waste, occupies little space (length and width) in the paint booth and can be installed in existing paint booths without requiring significant booth modification. It is also desirable to provide a painting apparatus wherein one painting robot is able to reach substantially all paintable surfaces on one side of the article to provide backup capability in the case of an inoperative robot.
Due to the conductivity of the waterborne paint, it is necessary to electrically isolate the grounded bulk paint supply system from a charged local dispensing canister and spray application system. In the prior art, the bell applicator, canister, canister drive, electrostatic cascade, and docking interface were all integrated into a single unit mounted on the robot wrist as shown in the U.S. Pat. No. 5,293,911 and the U.S. Pat. No. 5,367,944. Such an applicator had the following shortcomings:
1) The applicator is heavy, expensive, and subject to damage via collision with objects in the painting booth.
2) The applicator docking with a docking station must occur in a fixed booth position and therefore limits process flexibility.
3) The docking process takes cycle time as the robot must travel to and from the docked position. The canister filling can not start until the applicator reaches the docked position.
4) The docking hardware is expensive and unique to waterborne systems.
Another method to isolate the bulk paint supply system from the charged paint dispensing canister is to clean and dry the paint transfer line between the supply system and the canister. The cleaning and drying of the transfer line between the bulk supply system and the paint dispensing canister has not been successfully implemented in an automotive type painting system (rapid color changing on a continuous conveyance type system). There are several reasons why this type of system has not been used in the past. These shortcomings include:
1) The time to clean and dry the line and provide high voltage isolation exceeds the allotted dwell time between the vehicle bodies being painted.
2) The high voltage cannot be easily contained and thus the isolation system is compromised (arcing and pin holing). The electrostatic erosion can burn holes in the transfer line, the dispensing system, the supply line to the applicator, or the waste collection lines. The resultant down time would not be readily accepted for this type of application.
3) The amount of waste that is left in the paint transfer line is excessive when compared to other means of isolation.
4) The acceptable voltage limit does not provide high painting transfer efficiency as compared to other means of voltage isolation.